the palace. a terrifying scene in london for tourists at buckingham palace. the man with knives, the police who had to act fast. >>> the super bowl's superstar. which celebrity will win the play off the field? and will anyone ever be able to tackle betty white's moment? >> you're playing like betty white out there. >> that's not what your girlfriend says. >> baby! >>> from abc news world headquarters, this is "abc world news" with david muir. >>> good evening. thank you for being here on this sunday night. we begin this evening with that unfolding tragedy, a former navy s.e.a.l., a hero to members of the military. this evening authorities say he was targeted and killed by a fellow veter.